Usefulness of the whole-blood interferon-gamma release assay for diagnosis of extrapulmonary tuberculosis

The whole-blood interferon-gamma enzyme-linked immunosorbent assay (QuantiFERON-TB Gold [QFT-G]; Cellestis, Carnegie, Australia) has been studied mainly for diagnosing active pulmonary tuberculosis (TB) or latent TB. We prospectively evaluated its diagnostic usefulness in patients suspected with extrapulmonary TB (EP-TB). Of the 100 adult patients with suspected EP-TB, 43 were classified as “confirmed” EP-TB and 5 as “probable” EP-TB. Of the 48 with EP-TB, 27 (56%) were diagnosed with TB lymphadenitis and 11 (17%) with skeletal TB. The overall sensitivity and specificity of the assay were 69% (95% confidence interval [CI95], 53–81%) and 82% (CI95, 69–91%), respectively. Among 44 patients presented with cervical lymphadenopathy, the QFT-G assay showed 86% (CI95, 64–97%) sensitivity and 87% (CI95, 66–97%) specificity, whereas in 28 with skeletal involvement, the sensitivity and specificity of the assay were 45% (CI95, 17–77%) and 81% (CI95, 54–96%), respectively. These suboptimal diagnostic performances suggest that the QFT-G assay alone is not sufficient for the diagnosis of EP-TB.